1. Field of the Invention
The present invention relates to a photomask for fabricating a semiconductor device, and a method of fabricating the same, and more particularly, to a photomask on which phase gratings are formed for off-axis illumination (OAI), and a method of fabricating the same.
2. Description of the Related Art
Photomask images or patterns, which define various elements in a photolithographic process, are focused onto photoresist using light. In order to implement tiny features, finer images must be focused on the photoresist, and optical resolution must be increased. However, there are limits to the resolution which can be attained.
Thus, in order to fabricate a semiconductor device near the resolution limit of a photolithographic process, resolution enhancement techniques must be used. Resolution enhancement techniques include a method of using a light source having a wavelength smaller than that in prior art, a method of using a phase shift mask, and a method of using off-axis illumination (OAI), which is modified illumination.
Theoretically, in the case of using OAI, resolution is about 1.5 times higher than when using conventional illumination, and depth of focus (DOF) is also increased. As a semiconductor device becomes highly integrated, it is important to improve DOF, because there is always some unevenness on a wafer on which patterns are projected, due to preformed patterns or bending of the wafer, and the exposure of photoresist on the wafer surface or in all places of each chip is not performed on same focus surface.
One method of implementing OAI is to install in exposure equipment a modified aperture having an annular, dipole or quadripole optical transmission region, instead of a conventional aperture having a circular optical transmission region. In this method, the vertical component of incident light is cut off, and only a sloped component (that is, an off-axis component) reaches the photomask. In this case, the intensity of light emitted from the light source is reduced while passing through the modified aperture.
Another method is to attach an additional mask having phase gratings (hereinafter referred to as a grating mask) to the back surface of a photomask using a conventional aperture. In the method, light is diffracted by the phase gratings, so the vertical component of the light is offset and only an off-axis component is transmitted into a projection lens, and only light passing through the projection lens interferes with itself on a wafer on which the photoresist is coated, forming images from the light. The intensity of light is not reduced as when using the modified aperture, but problems may occur when the grating mask is attached to the back surface of the photomask. Various losses and uncontrollable reasonable factors, such as that the grating mask must be detached and reattached for cleaning after being used for a certain time, are inherent in this method, and thus, it is practically impossible to employ the method in mass production.
FIGS. 1 and 2 illustrate a conventional grating mask, showing the illumination pattern on the right corresponding to the shape of the grating mask on the left. For reference, the simulation tool used in the present invention was SOLID-C. Bright portions in the drawings illustrating the illumination shape are optical transmission regions, and dark portions are optical cutoff regions.
FIG. 1 illustrates a grating mask 10 on which a phase grating 5 comprising an alternating pattern of lines and spaces is formed at a constant interval, and the grating mask 10 phase-shifts incident light by 180°. In this case, illumination having a dipole shape is implemented, as shown in the right of FIG. 1. The illumination having a dipole shape has a different illumination effect on patterns in a horizontal or vertical direction, and thus is effective when line & space patterns are transferred onto a wafer.
FIG. 2 illustrates a grating mask 20 on which a checkered phase grating 15 is formed, to phase-shift incident light by 180°. In this case, illumination having a quadripole shape is implemented, as shown in the right of FIG. 2. The illumination having a quadripole shape has the same modified illumination effect on patterns in horizontal and vertical directions. Thus, the illumination having a quadripole shape is used to effectively transfer isolated patterns.
However, when patterns transferred onto the wafer have pitches in an x-direction, a y-direction, and a diagonal direction, at different intervals, it is very difficult to form patterns in the x and y-directions and the diagonal direction under only one illumination condition. There is very restricted ability to obtain a desired critical dimension (CD) with respect to the directions, and thus, it is impossible to obtain a process margin. In order to obtain the process margin, illumination conditions that are suitable for various pattern shapes are required.
In addition, in the conventional OAI method, the OAI limit of the exposure equipment is determined by a numerical aperture (NA) of a condenser lens included in the exposure equipment. However, as design rules quickly diminish, it becomes difficult to obtain the desired process margin from the OAI of the exposure equipment. In this case, the only options are to find new techniques or to use exposure equipment having a higher NA. However, it is far cheaper to use conventional exposure equipment having lower NA. In this case, since it is very difficult under the present circumstances to obtain process capabilities through the resolution enhancement technique, a method is required by which the OAI limit of the exposure equipment can be overcome.